Monitoring of powder formation via optical emission spectroscopy and self-bias-voltage measurements for high depletion μc-Si:H deposition regimes

摘要

Microcrystalline silicon fabricated by plasma-enhanced chemical vapour deposition (PECVD) is commonly used as an absorber material in thin-film tandem solar cells. The source gases used in the μc-Si:H PECVD process are silane and hydrogen. One way to further increase the production efficiency of solar modules is to increase the gas utilization during deposition of the silicon absorber layer. In this work this is achieved by reducing the hydrogen flow. These deposition conditions are known to promote powder formation in the plasma, which can be detrimental for the solar cell's conversion efficiency as well as for the maintenance of the system. Therefore, an easily applicable approach to determine powder formation in-situ during the PECVD process is presented. Both the self-bias-voltage and the ratio of the optical emissions from SiH* to H_β as function of the gas residence time in the plasma is used to determine the onset of powder formation. Furthermore, a clear link between the precursor gas residence time in the plasma to the onset of powder formation is shown independent of the chosen pressure.